The present invention relates to a microminiature force-sensitive switch capable of being trimmed to detect changes in an external condition.
Microminiature switches responsive to changes in external conditions are known in the art, as exemplified by U.S. Pat. No. 4,543,457 to Petersen. The switch therein described includes a silicon wafer having a reduced-thickness deflectable membrane which moves in response to changes in external conditions and thereby establishes contact between a common terminal and first one then progressively more confronting terminals of the switch device.
The Petersen apparatus teaches one embodiment of the switch in which the deflectable member takes the form of a diaphragm etched in a silicon substrate which is adapted to bulge from its relaxed state to a more strained state in response to an increase in an applied external force. This force may comprise a pressure differential, which requires that one side of the diaphragm remain at a constant pressure. This can be accomplished by forming a hermetically sealed chamber around this side of the diaphragm. When the switch is to be used to measure acceleration, a mass may be attached to the diaphragm. The diaphragm also may be modified to include a metal layer having a substantially different thermal coefficient of expansion than that of the diaphragm to enable the measurement of temperature changes.
Another embodiment of the switch taught by Petersen is an elongated beam anchored at one or both ends which deflects in a manner analogous to the function of the diaphragm in digitizing or monitoring temperature or acceleration.
It is generally known in the art that switching devices may be trimmed so as to set the threshold level of detection. Trimming has been accomplished by chemical and laser techniques and is a major component of the total cost of trimmed devices. The trimming techniques generally use a plurality of external taps or pads for accessing the switching devices, and depend for their effectiveness on the closing of switches in a particular order for each individual device.
The prior art described above has not overcome several problems that are addressed and solved by the present invention. For example, Petersen teaches a switching device that is responsive to changes in external conditions such as temperature, pressure or acceleration, yet the fabrication and operation of the device to detect a particular threshold level of the changing external condition is not achievable in the most efficient and cost effective manner due to structural limitations in the Petersen apparatus. Specifically, leads must be extended from each switch to external taps or pads in order to calibrate the device. As a preselected force is applied to the deflectable member, each switch must be monitored for closure in order to determine which switch closes at a desired deflection level. Such a necessary procedure is time consuming and labor-intensive, which increases the cost of each device and renders the device unsuitable for mass production.
The cost is also raised by the extra materials and parts required and by the more frequent failure of the hermetic seal due to the need for penetration of the seal by multiple switch leads. Moreover, because such devices require more switch leads and taps or pads, the number of switches on each device is physically limited. This limits the achievable precision of detection and increases the user's cost by requiring the purchase of more devices to accomplish a given operation. Furthermore, the probability of failure of a device increases with increasing complexity and with the addition of elements. The extra lead, taps, etc., also increase the physical size of each device, which limits its application.
Although the use of fusible links in an integrated circuit is known, such use has generally been limited to applications involving read only memory elements or the fixing of an impedance value. In U.S. Pat. No. 4,016,483 to Rudin, for example, the impedance value in a microminiature integrated circuit by the selective blowing of fuses is shown. The device therein described includes fusible aluminum links in parallel with binary weighted resistive elements. The application of electrical energy to selected fusible links causes them to open, thereby inserting selected resistive elements into the impedance circuit.
The prior art does not teach the use of fusible links to enable trimming of a switching level, nor such a use that does not depend on the designer-intended order of operation of the trimming switches.
To overcome these limitations and disadvantages of the prior art, the present invention combines a deflectable member, a plurality of switches, and fusible links in such a way so as to produce a device which is compact, less costly to produce, and which can be trimmed or calibrated after the device has been fabricated without regard to the order of operation of the switching elements of the device as set by the design and tooling and without requiring the leads from each switch to be available for external connection during this calibration process.
It is therefore a general object of the present invention to provide a microminiature switching device for detecting or monitoring of a threshold level of an external condition such as pressure, temperature or acceleration, the switching device being designed so that the threshold level to be detected or monitored is easily and accurately calibrated and so that the device is reliable in operation subsequent to its calibration.
Another object of this invention is to reduce the cost of manufacturing and trimming such a switching device.
A further object of this invention is to provide a method for trimming the switching device that is independent of the order of operation of the switches of the device.
A more specific object of the invention is to provide a microminiature switching device whose elements are contained within a hermetically sealed chamber.
Another object of this invention is to minimize external connections to the elements of the switching device to minimize the number of required penetrations of the hermetic seal so as to reduce the possibility of seal failure.
The present invention achieves these objects by providing a reliable, low-cost, microminiature force-sensitive switching device which is capable of being easily trimmed without the necessity of a plurality of additional external connections to the apparatus and without regard to the operational order of its elements, resulting in a device which accurately and reliably senses a selected external condition within a desired range.
According to the present invention, the switching device includes a deflectable member whose deflection is caused by changes in an external condition to be detected, such as temperature, pressure or acceleration. Movement of the deflectable member from a less strained to a more strained condition causes the establishment of electrical contact between a common contact and first one then progressively more switch terminals in the device. The circuit is completed by the series connection of each switch to a fusible link whose other terminal is tied in common with all other fusible links in the switching device which is in a common terminal. The common terminal connected to the fusible links is, in the preferred embodiment, within the chamber formed by the hermetic sealing of a first substrate, including the deflectable member, and a second substrate. From the common terminal, a single conductor is passed through the seal to enable electrical connection of the device to an external calibrating or monitoring circuit. This reduction in the number of seal penetrations reduces the seal failure rate.
The trimming of the device to preselect the threshold value of the external condition to be detected is accomplished easily, accurately and reliably. A chosen calibrating level of the external condition is applied to the device. The resultant force causes the deflection of the deflectable member and consequent closing of one or more switches. A voltage is then applied to the common terminals of the device, causing a current to pass through the closed switches and the fusible links connected in series with them. These fusible links are permanently blown, while the fusible links connected in series with the switches which did not close on application of the calibrating condition are not blown.
The result is that the switches whose fusible links were open-circuited can no longer be connected to the common terminal on subsequent application of a force on the deflectable member. Only at a level of force greater than the threshold defined by the externally applied calibrating condition will the deflectable member move sufficiently to close an additional one or more switches to create a conductive path between the external terminals and thereby enable the generation of an electrical signal indicative of the level of the external condition exceeding this threshold level. By connecting each switch in said switching device to a fusible link, it is possible automatically to trim the device, enabling multiples of the switching devices to be trimmed at once, without requiring identification of the particular switches that are closed or open at any level of the external condition.
These and other objects, features and advantages of the present invention will become more apparent to those skilled in the art from the following detailed description of the invention in conjunction with the accompanying drawings.